The present invention relates to a valve operating device for internal combustion engines, and particularly to a valve operating device having a hydraulic valve operation mode changing mechanism for changing the operation mode in which an intake valve or an exhaust valve is opened and closed between a low-speed mode, corresponding to low-speed operation of the engine, and a high-speed mode, corresponding to high-speed operation of the engine, and control means for controlling operation of the valve operation mode changing mechanism according to the rotational speed of the engine.
Valve operating devices of the type described above are known, one example being disclosed in Japanese Laid-Open Publication No. 61-19911. With such conventional arrangements, operation of the valve operation mode changing mechanism is controlled by controlling hydraulic pressure according to the rotational speed of the engine. When the viscosity of working oil is high, as at low temperatures, however, the valve operation mode changing mechanism of such arrangements cannot operate quickly to vary the hydraulic pressure for changing the operation mode of the intake or exhaust valve from the low-speed mode to the high-speed mode. Under this condition, regardless of a high-speed operation of the engine, the intake or exhaust valves may remain in the low-speed mode. When this occurs, mechanical problems, such as, for example, a jump of the intake or exhaust valve may occur due to the resiliency of the spring of a lost-motion mechanism. Moreover, where the valve operating device is incorporated in an engine having an electronic fuel injection device of the intake vacuum/engine speed type and a spark advancer, the air-fuel mixture may become too rich, or the ignition spark may be retarded excessively.